Glideslope position detection system for use with an independent aircraft landing monitor

ABSTRACT

An glideslope position detection system wherein a pair of passive reflectors are located in a spaced apart configuration at a known position adjacent an airport runway. Each of the reflectors are shaped to direct different reflection patterns in response to radar signals transmitted from an aircraft during a glideslope approach to the runway. One of the reflectors directs a reflection pattern beneath the glideslope and the other of the reflectors directs a radar reflection pattern above the glideslope, the reflection patterns providing equal amplitude radar returns to an aircraft traveling along the glideslope. The aircraft carries a radar system for detecting the relative radar return signal amplitudes from the two reflectors. Circuitry is responsive to comparison of the radar return signal amplitudes from the reflectors to generate indications of the position of the aircraft relative to the runway glideslope.

Uite States Patent [191 Asam [ 51 Feb. 13, 1973 [75] Inventor: Edward F.Asam, Dallas, Tex.

[73] Assignee: Texas Instruments Dallas, Tex.

22 Filed: July 15,1970

21 Appl.No.: 55,164

Incorporated,

[52] US. Cl ..343l5 LS, 343/7.3, 343/18 B,

[51] Int.Cl ..G01s 9/16,H0lq 15/14 [58] Field of Search .343/5 R, 5 LS,5 SL, 7 A, 7 TA, 343/73, 11 R, 13 R, 18 B, 18 L, 108 R 2,644,155 6/1953Emmett ..343/5 LS Primary ExaminerStephen C. Bentley Att0rney.lames 0.Dixon, Andrew M. Hassell,

Harold Levine and Rene E. Grossman [57] ABSTRACT An glideslope positiondetection system wherein a pair of passive reflectors are located in aspaced apart configuration at a known position adjacent an airportrunway. Each of the reflectors are shaped to direct different reflectionpatterns in response to radar signals transmitted from an aircraftduring a glideslope approach to the runway. One of the reflectorsdirects a reflection pattern beneath the glideslope and the other of thereflectors directs a radar reflection pattern above the glideslope, thereflection patterns providing equal amplitude radar returns to anaircraft traveling along the glideslope. The aircraft carries a radarsystem for detecting the relative radar return signal amplitudes fromthe two reflectors. Circuitry is responsive to comparison of the radarreturn signal amplitudes from the reflectors to generate indications ofthe position of the aircraft relative to the runway glideslope.

13 Claims, 6 Drawing Figures PATENTEDFEB I 31975 SHEET 10F 3 FIG.

FIG 2 RANGE BINS FIG. 3

PATENTEO FEB I 31975 VIDEO SHEET 2 0F 3 INPUT CFAR MATcHED HIGH PASSFILTER i FALSE ALARM cOuNTER L5O 55/ REFERENCE v J 54 [7O SEVEN BIT 67WSHIFT REGIsTER REFERENCE v THRESHOLD cIRcuIT DEcODED VIDEO AcOuIsITIONPULSE /78 THRESHOLD T INTEGRATOR AND DELAY 86 I TIME SAMPLE 94 RANGEDIscRIMINATOR AND HOLD F'LTER RATE DELAY kso 92) as AcOuIsITION GATELOCK vOLTAGE To ON PULSE RATE p DOWN CONVERTER RANGE GATE 96 g ggDIGITAL INTEGRATOR RANGE UPPER LOwER 9a LOBE LOBE [00 D/A ANALOG TcONvERTER RANGE RANGE RANGE SQQZ COMPARATOR T RAMP PMT GENERATORPATENIED FEB 1 31m SHEET 3 OF 3 DECODED VIDEO UPPER LOBE SAMPLE PULSE fi/30 PEAK DETECTOR AND HOLD PEAK I DETECTOR AND HOLD VIDEO IN FILTERSAMPLE AND HOLD ERROR SIGNAL GLIDESLOPE POSITION DETECTION SYSTEM FORUSE WITH AN INDEPENDENT AIRCRAFT LANDING MONITOR This invention relatesto airborne radar systems, and more particularly to a glideslopedetection system for use with an independent aircraft radar landingmonitor system.

A number of aircraft guidance systems have previously been developed forassisting the landing of aircraft. However, the advent of large, highspeed jet passenger aircraft, and the development of automatic landingsystems such as flight director systems, have resulted in the need foran onboard aircraft landing monitor which is independent of ground basedelectronic guidance equipment to enable the pilot to progressivelymonitor the final aircraft approach, touchdown and rollout.Specifically, a need has arisen for an onboard independent landingmonitor which provides the pilot with positive assurance that theaircraft localizer approach is valid, that the aircrafts true positionrelative to the runway center line and threshold is satisfactory andthat the airport runway is clear of obstructions. Additionally, it isdesirable to provide an independent indication of the range and rangerate to touchdown on the airport runway, as well as an indication of thevertical position of the aircraft relative to the desired glideslopeapproach. Such an independent landing monitor is particularly desirableduring blind or low visibility aircraft takeoffs or landings, and duringlandings at airports which are unequipped or underequipped withnavigational aids.

In accordance with the present invention, a short range, high resolutionmapping radar system is located onboard an aircraft and is independentof ground based electronic equipment to monitor runway alignment and thelike during approach, touchdown and rollout phases of aircraft landing.The present system utilizes a high resolution antenna system incombination with a visual radar display which presents a real-worlddisplay of the approaching runway to the pilot on a oneto-onecorrespondence to real-world perspective. A range tracker system sensescoded reflections from reflectors adjacent the airport runway to provideindications of range and range rate to touchdown. A glideslope detectionsystem senses the relative amplitudes of radar returns from two spacedapart reflectors in order to generate an indication of the position ofthe aircraft relative to the glideslope. The pilot may then accuratelyidentify the runway threshold, accurately determine the angle to centerof the runway, measure lateral offset, determine range and range rate totouch down, determine the relative glideslope position of the aircraft,and make a smooth transition to visual runway information.

In further accordance with the present invention, an aircraft glideslopedetection system includes a radar system carried onboard an aircraft fordetection of reflected radar signals from a plurality of reflectorsspaced along an airport runway. Each of the reflectors have reflectionpatterns of different angles in the elevation. Circuitry is providedonboard the aircraft for comparing the amplitudes of the reflected radarsignals from the reflectors. Circuitry is responsive to the comparingcircuitry for generating indications of the vertical position of theaircraft with respect to the desired glideslope.

In accordance with another aspect of the invention, at least tworeflectors are located in a spaced apart configuration adjacent anairport runway. One of the reflectors is shaped to direct a first radarreflection pattern beneath the runway glideslope and the other of thereflectors is shaped to direct a second radar reflection pattern abovethe runway glideslope. The first and second reflection patterns provideequal amplitude radar returns along the runway glideslope. A radarsystem is carried onboard an aircraft for detecting the relative radarreturn signal amplitudes of the first and second reflection patternsduring glideslope approach by the aircraft. Circuitry is responsive tothe radar system for generating an indication of the position of theaircraft relative to the glideslope.

For a more complete understanding of the present invention and forfurther objects and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic illustration of the elevational antennaradiation pattern of the invention during glideslope approach;

FIG. 2 is a diagrammatic illustration of the azimuth sweep of thepresent antenna radiation pattern and the placement of reflectorsaccording to a pseudorandom coded configuration adjacent the runway;

FIG. 3 is a diagrammatic illustration of the installation of the presentlanding monitor radar system in the nose of an aircraft;

FIG. 4 is a block diagram of the preferred embodiment of the rangetracking system of the invention;

FIG. 5 is a diagrammatic illustration of the respective radiationpatterns provided by the glideslope reflectors of the invention; and,

FIG. 6 is a block diagram of the preferred glideslope detectioncircuitry of the invention.

Referring to the drawings, FIG. 1 illustrates an aircraft 10 in aconventional glideslope approach attitude to an airport runway 12. Theglideslope 14 will typically have an angle in the range of 2.5 to 30.According to the preferred embodiment of the invention, a radar beam istransmitted from the nose portion of the aircraft 10 with an elevationalantenna beamwidth of about 17. The antenna beamwidth is verticallycentered about the glideslope 14 in order to enable high resolutionradar mapping of the approaching runway 12 during landing approach ofthe aircraft 10. In the preferred embodiment of the invention, a maximumrange of about five miles, with a range of about two miles for initialrunway acquisition, is afforded to the landing monitor radar systemcarried by aircraft 10. Aircraft 10, landing with typical glideslopeangles will thus be about 2,600 feet from the end of the runway 12 at analtitude of 200 feet and about 1,300 feet from the runway 12 at adecision altitude of feet.

As shown in FIG. 2, the aircraft 10 transmits an antenna beam having anazimuth beamwidth of approximately 03 to 0.4". The antenna beamwidth iscontinuously swept over an azimuth sweep angle of approximately 30. Inthe preferred embodiment of the invention, the radar antenna aboardaircraft 10 is swept 15 on either side of the aircraft center line at ascan rate of about 2.5 Hz. The radar system thus looks at the runwayarea five times per second, since the beamwidth is about 04, and sincethe pulse repetition rate in the preferred embodiment is 15,000 pulsesper second, each point target results in about 21 radar returns persweep. Hence, the present radar receiver receives bursts of radarreturns at about 0.2 second intervals. The present airborne radar systemis thus able to update the range data, in the manner to be laterdescribed, every 0.2 seconds, and the information utilized to updatethis data is based on at least 21 radar returns.

FIG. 3 is a partially cut-away view of the nose portion of aconventional aircraft which illustrates the basic components of thepresent radar landing monitor system. A mechanically swept antenna 18 ismounted in the nose of the aircraft and comprises in the preferredembodiment an elongated resonant edge slotted waveguide array which isreciprocated about a vertical axis 20. Pulsed radar signals aretransmitted via the antenna 18 through suitable waveguide connections 22which extend from a transmitter-receiver housing 24. The present rangetracking circuitry of the invention, to be later described in greaterdetail, is also located in housing 24. Sweep generator circuitry islocated in housing 26 and provides electrical signals via leads 28 to adisplay monitor radarscope 30 mounted in the instrument panel of theaircraft. Display monitor radarscope 30 is preferably of the direct viewstorage tube type and provides a display surface 32 wherein the pilotmay receive a real-world perspective indication of the upcoming runway12. Display monitor radarscope 30 includes various adjustments andselect knobs to provide alternate conventional B-sweep or PPI displaysweep modes, if desired. Power supply circuitry for the system iscontained within a housing 34.

The present system provides a practical independent landing monitorsystem for use with such large passenger capability superjet aircraft asthe Lockheed LlOll transport aircraft or the DC-lO aircraft. The presenthigh resolution rate radar system permits visual assessment to the pilotof the aircrafts alignment with the runway center line during the finalphase of approach, and provides accurate range information to touchdown,even in the event of inclement weather such as rain, snow or fog whichprovides zero-zero visibility.

For a more detailed description of the antenna 18, reference is made tothe copending patent application, Ser. No. 054,990, filed July 15, 1970,and entitled Mechanically Swept Radar Antenna For Use With An AircraftLanding Monitor System". For more detailed information of the overallindependent landing monitor radar system of the invention, reference ismade to the copending patent application, Ser. No. 054,979, filed July15, 1970, and entitled Independent Aircraft Landing Monitor System. Adetailed description of the real-world display system of the invention,wherein a display of the approaching runway is provided with real-worldlinear perspective is found in copending patent application, Ser. No.055,166, filed July 15, 1970, and entitled Real-World PerspectiveDisplay For Use With An Independent Aircraft Landing Monitor System.While the above noted independent landing monitor radar system isdesirable for use with the present range tracking system, it will beunderstood that other airborne radar systems could be utilized inconjunction with the present radar range tracking system if desired.

Referring again to FIGS. 1 and 2, an important aspect of the presentrange tracking system is the provision of a plurality of radarreflectors 40, 42 and 44 "along each side of the airport runway 12.Reflectors 40-44, which preferably are corner reflectors which act asideal point targets, are arranged in a coded configuration in order toenhance the detection thereof by the airborne radar system. In thepreferred embodiment, the reflectors 40, 42 and 44 are arranged in aspaced apart coded configuration in order to modulate the return radarvideo signal with a pseudorandom digital code. When the radar receiverlocated onboard aircraft l0 detects the presence of the predeterminedpseudorandom code in the radar return, the present radar tracker is ableto lock on and track the range of the coded corner reflectors 40-44 withgreat accuracy.

In the preferred embodiment of the invention, the coded reflectors 40-44are disposed along each side of the runway 12 along an interval A whichmay comprise for instance, a length of about 350 feet. The interval A isdivided into seven range bins, and the presence or absence of a cornerreflector in each of the range bins corresponds to either a logic one ora logic zero in the predetermined pseudorandom code. In the preferredembodiment of the invention, a seven bit Barker pseudorandom code isutilized which has the form of l lOl000.The resolution of the presentradar system dictates a range bin dimension of about 50 feet in length,so the entire reflector array, including vacant range bins for logiczeros," occupies about 350 feet along each side of the runway 12. Therange bins along dimension A terminate at the desired point of touchdownon runway 12, which may be for instance about 1,000 feet from the end ofthe runway.

The multiple radar reflectors 42 and 44 are utilized in the second andfourth range bins in order to insure that all three of the logic ones ofthe predetermined code simultaneously appear within the narrowtransmitted radar beam as the look angle of the radar system changeswith respect to the corner reflectors. Thus, as the aircraft l0approaches the runway 12, the swept radar beam will alwayssimultaneously sense the logic ones in the predetermined code even asthe look angle continuously varies.

Pseudorandom codes are preferably utilized with the present reflectorconfigurations, due to the fact that such codes provide highautocorrelation functions at code coincidence, thus enabling extremelyhigh certainty of detection of the coded reflectors along the runway. Asis known, such pseudorandom codes are distinguished by the fact that theprobability of each portion of the code being a one or a zero is equal,and due to the fact that the autocorrelation function is generally equalto zero except when identity or coincidence is obtained. For a moredetailed description of pseudorandom codes, reference is made to ModernRadar, by Berkowitz, Chapter 4, published 1965 by Wiley & Sons, Inc; Forfurther information on Barker codes, reference is made to GroupSynchronizing of Binary Digital Systems, Communication Theory, by R. H.Barker et al, New York, Academy Press, Chapter 19, 1953.

Although a seven bit Barker pseudorandom code has been illustrated inthe preferred embodiment, it will be understood that other binary codescould be utilized to provide varying degrees of resolution according todifferent desired operating characteristics of the system.

FIG. 4 illustrates a block diagram of the preferred embodiment of therange tracker circuitry of the invention. A radar video input signalgenerated by the radar receiver circuitry in housing 24 is fed to amatched high pass filter 50. This video input is preferably generated bythe receiver circuitry described in the previously identified copendingpatent application, Ser. No. 054,979, filed July 15, 1970, entitledIndependent Aircraft Landing Monitor System, but may alternatively begenerated by other known types of pulse radar systems wherein videopulses representative of return radar echoes are developed from thereturn radar signals.

The matched high pass filter 50 is matched to the expected receivedsquare pulse from the corner reflectors 4044 and thus the filter 50tends to pass radar returns from the point targets and to attenuatereturns from distributed targets. Filter 50 is thus matched to maximizethe signal-to-noise ratio between the expected radar returns from thecoded reflectors and between ground noise and clutter. The filteredsignal is fed to a comparator circuit 52, wherein a comparison is madeof the magnitude of the filtered signal and a threshold level signalsupplied on lead 54. Comparator 52 generates a logic 1 output wheneverthe filtered video signal exceeds the reference threshold appearing onlead 54. The threshold level signal is varied, in a manner to besubsequently described, to provide a constant false alarm rate (CFAR).

The output from the comparator 52 is fed to a false alarm countercircuit 56, which is a range gated integrator operated in response to aCFAR gate signal supplied via a lead 58. The output of this integratoris a voltage proportional to the number of threshold crossings whichoccurred during the length of the CFAR gate pulse. The resulting signalfed from a counter 56 is fed to a summing network 60, wherein the signalis algebraically summed with a voltage from a reference voltage source62. The voltage level of reference source 62 is proportional to thedesired number of false alarms. The resulting output signal from thesumming circuit 60 is an error signal which is applied to an amplifier64. The amplified error signal is fed back through lead 54 to thecomparator 52 as the CFAR threshold level signal. This threshold levelsignal is raised or lowered depending on the polarity of the referencesignal, to provide the same number of false alarms, which are caused bynoise or clutter, for the system, independent of the various reflectioncharacteristics of a particular airport.

The CFAR threshold signal is also applied to a summing point 66. Avoltage from a reference voltage generator 67 is also applied to thesumming point 66 and the resulting signal is applied to an input of avoltage comparator circuit 68. The voltage level at the output ofsumming point 66 is suchthat the probability of false alarm ismaintained below a specified maximum. The video input is applied to thesecond input of the comparator 68, and the resulting output is appliedto a seven bit shift register 70. The output of comparator 68 is a logicone when the input video is above the voltage threshold, and theinformation is shifted into shift register 70.

Each output of the shift register is connected through a weightingfunction circuit 72 and then applied to a summing network 74. It will beunderstood that the shift register 70 and the summing network 74comprise a seven bit correlation circuit. The shift register 70 may, forinstance, comprise seven flipflop circuits each generating a +1 or Ivoltage signal, dependent upon whether or not the digital informationstored in a particular flipflop is in agreement with the correspondingbit in the desired Barker code. Thus, the register 70 compares thereceived radar returns with a stored representation of the predeterminedpseudoran I dom code. Upon summation of the outputs of the shiftregister 70 at the summing circuit 74, the resulting voltage indicatesthe correlation of the stored signal within the shift register 70 withthe predetermined Barker code being utilized.

As an example, with the use of the configuration shown in FIG. 2 of thereflectors 40, 42 and 44, when a digital signal 1 101000 is stored inthe shift register 70, a high voltage indication is generated by thesumming circuit 74 and applied to a threshold circuit 76. If a differentcode is stored in the shift register 70, the output from the summingcircuit 74 would be substantially lower due to the definition of apseudorandom code. The correlation circuitry utilized with the presentinvention thus provides a very high probability of detection of thecoded configuration of reflectors when a high signal is applied from thesumming network 74 to the threshold circuit 76. Threshold circuit 76passes only signals above a predetermined threshold to prevent operationof the circuit until a particular coded configuration of reflectorsadjacent a runway airport are sensed. The magnitude of the thresholdcircuit 76 is set to obtain a desired probability of detection for thecircuit. It will be understood that other types of correlation circuitscould be utilized for detection of the predetermined code of theinvention, and for further reference to such correlation circuits,reference is made to Radar Detection, by DiFranco and Rubin, 1968,Section V, Prentice-HalL The output of the threshold circuit 76 istermed the decoded video signal and is applied to a gated pulseintegrator 78. The integrator 78 is gated by an acquisition gategenerated by the range tracker circuitry to be subsequently described.Whenever the acquisition gate and the output from the correlationcircuit are coincident, a given voltage is added to the pulse integratorcircuit. Thus, if a sufficient number of pulses arrive at the pulseintegrator 78 within a predetermined period of time, the outputgenerated by the integrator 78 will exceed an acquisition thresholdvoltage set within the acquisition threshold and delay circuit 80. Thishigh output from the integrator 78 indicates that a valid target hasbeen decoded by the system, and a lock-on signal is generated after apredetermined delay from the acquisition threshold and delay circuit 82and applied to the voltage to pulse rate convertor 82. The lock'onsignal indicates that the range computer of the system is tracking thepoint of touchdown on the airport runway 12. If two scan periods of thesystem pass without detection of a threshold crossing by the circuit 80,the system drops out of the track mode and the target search mode of thesystem resumes.

The range tracker portion of the present system is an approximate typeII servo mechanism tracking loop in the track mode of operation, and atype I servo mechanism during acquisition mode. The decoded video signalis applied to a time discriminator circuit 86, and also to a delay 88.The output of the time discriminator circuit 86 is proportional to thetime differential between the decoded video and the range gate. Theoutputs of the time discriminator 86 and of the delay 88 are applied toa sample and hold circuit 90. The sample and hold circuit 90 comprises agated amplifier which utilizes a storage capacitor for the samplingfunction. FET devices are utilized within the gated amplifier to givehigh impedance isolation. The output of the sample and hold circuit 90is applied through a low pass filter 92 wherein the cutoff frequency ofthe system is optimized. The output of the filter 92 is applied throughan amplifier 94 which generates an output signal indicative of the rangerate of the system during lock-on tracking mode operation.

The output of the filter 92 is also applied to a voltage to pulse rateconvertor 82 which is operated in dependence upon the lock-on signalgenerated by the acquisition threshold and delay circuit 80. The outputof the pulse rate convertor 82 is a pulse train with a pulse rateproportional to the magnitude of the filtered and amplified errorvoltage of the system. The output of the convertor 82 is applied to abinary coded decimal digital range integrator circuit 96 which in turngenerates a digital output indicative of the range being tracked by thesystem. An up-down count signal is also applied from the converter 82for operation of the range integrator 96. The range integrator 96 maycomprise any one of a number of suitable conventional circuits, such asthe SN54192 or SN74192 circuits manufactured and sold by TexasInstruments Incorporated.

The output of the range integrator 96 is fed to a digital to analogconvertor which generates an analog indication of the range beingtracked by the circuit. The output of the convertor 98 is also fed to arange comparator circuit 100. A ramp voltage is also applied to an inputof the range comparator 100 by a ramp generator 102 which is controlledby the premaster trigger (PMT) of the radar system. The comparator 100generates a pulse at time coincidence when the ramp voltage equals theinternal range voltage generated by the convertor 98. The output fromthe range comparator 100 is fed to the range gate generator 104, whichgenerates the acquisition gate applied to the pulse integrator 78 andalso which generates the CFAR gate applied via lead 58 to the falsealarm counter 56 and the range gate applied to the time discriminator86.

In operation of the range tracking circuitry during the search mode ofoperation, the lock-on signal applied to the voltage to pulse rateconvertor 82 is at logic zero. This places a bias at the input to thevoltage to pulse rate convertor 82 which causes the range integrator 96to count from minimum to maximum range in a predetermined sweepinterval. The digital signal from the range integrator 96 is convertedto a proportional analog voltage by the convertor 98. The resultinginternal range voltage then sweeps repeatedly from a minimum value to amaximum value until lock-on occurs.

The premaster trigger (PMT) initiates the generation of a ramp voltageeach time a pulse is transmitted by the radar system. At the time whenthe ramp voltage and internal range voltage are equal, the output of therange comparator changes from a logic 0 to a logic I. This transitiontriggers the CFAR gate from the range gate generator 104, as well astriggers the generation of the acquisition gate and the range gatetherefrom. These gates sweep from minimum range to maximum range untilthe target decoder portion of the circuitry detects the presence of theBarker code in the reflected radar video. This causes the lock-on signalto go from logic 0 to logic 1, and places the tracker system in thetrack mode of operation.

When the present range tracker is in the track mode of operation, eachtime the predetermined reflector target is detected, the timediscriminator circuit 86 generates an error signal whose magnitude andpolarity are proportional to the time error between the range gate andthe decoded video signal. The delayed video signal from the delay 88triggers the sample and hold circuit 90, which then holds the generatedtime error until the next reflected video pulse is received. Thebandwidth of the sample and hold circuit is such that it averages theerror over the entire burst of twenty or more radar returns per scan ofthe radar system. Consequently, the tracking loop treats each burst ofdecoded pulses as a single sample. The amplified and filtered errorsignal from the filter 92 is thus proportional to the range rate betweenthe aircraft and the point of touchdown. This voltage is also utilizedto update the range integrator 96 and subsequently to position thevarious range gates of the circuitry.

Referring to FIG. 5, the glideslope 14 of a conventional airport runwayis illustrated. A pair of passive radar reflectors and 122 are locatedadjacent the runway a distance B apart. During glideslope approach tothe runway, the radar antenna in the aircraft sweeps across the passivereflectors 120 and 122 in the manner shown in FIGS. 1 and 2. Reflector120 is located the greatest distance from the approaching aircraft andthe reflector directs a reflected radiation pattern 12d generally abovethe glideslope 14 in the elevation plane. The reflector 122 reflectsradar signals transmitted from the approaching aircraft along aradiation pattern 126 which is directed generally below the glideslopepath 14 in the elevational plane. The reflected radiation patterns 124and 126 overlap in an area 128 along a substantial extent of theglideslope 14, thus providing an equal amplitude reflection from each ofthe reflectors 120 and 122 when the aircraft is traveling along theglideslope path 14.

However, when the aircraft is traveling substantially above theglideslope path 14, it will be understood that the reflected signalamplitude received by the aircraft from reflector 120 is substantiallygreater than the reflected signals received from the reflector 122.Conversely, when the aircraft is traveling below the glideslope path 14,the amplitude of the reflected radar signals received from reflector 122will be substantially greater than the reflected signals received fromthe reflector 120. The present system, in conjunction with the rangetracker shown in FIG. 4, determines the relative amplitudes of thereflected signals from reflectors 120 and 122 to determine the verticalposition of the aircraft with respect to the glideslope path 14.

It will be understood that the reflectors 120 and 122 may comprise twoof the reflectors 40-44 described in FIG. 2. Alternatively, thereflectors 120 and 122 may be located some distance away from thepseudorandom coded reflectors 4044. In any case, however, the reflectors120 and 122 will be referenced with respect to the position of thepseudorandom coded reflectors 40-44 in order that the present rangetracking system may accurately determine the exact location ofreflectors 120 and 122. The distance B separating the reflectors 120 and122 will be determined by the resolution of the airborne radar system.In the preferred embodiment, the distance B is in the range of aboutfifty feet and the reflectors 120 and 122 have a radar cross section inthe neighborhood of about 1,000 square meters. In the preferredembodiment, the reflectors are physically about 3 feet high by about 2feet wide. The actual dimensions and particular configuration of thereflectors will vary according to various desired operatingcharacteristics of the system, but the reflectors will normally be ofthe corner reflecting type safe to provide radiation patterns accordingto FIG. 5 with respect to the glideslope path. The reflected beam widthsfrom the reflectors are generally wide in order to provide a largeregion in which valid glideslope error information is available with thepresent system. The cutoff portion of the radiation pattern from eachreflector is sufficiently sharp so as to provide angular accuracy in therange of about one-tenth of one degree.

FIG. 6 is a block diagram of the glideslope detection circuitry of theinvention. The video signal input is applied to the inputs of peakdetector and hold circuits 130 and 132. The video input signal isderived from the video input signal applied to the range trackercircuitry shown in FIG. 4, and is generated by the radar receivingcircuitry carried in the nose of the aircraft 10 as previouslydescribed. An upper lobe sample pulse is generated by the range gategenerator 104 shown in FIG. 4 and applied to peak detector 130 tocontrol the sampling of the reflected signals from the reflector 120.The generation of the upper lobe sample pulse is obtained from the rangetracker circuitry shown in FIG. 4 due to the fact that the location ofreflector 120 relative to the pseudorandom coded reflectors 40-44 ispredetermined.

Additionally, a lower lobe sample pulse is generated by the range gategenerator 104 in order to enable sam' pling of the radar reflectionsfrom the reflector 122. This sample pulse is applied to the peakdetector and hold circuit 132 for proper time sampling of the videoinput signal. The peak detector and hold circuits 130 and 132 sample andhold the highest maximum amplitude of the video input during the sampletime. The two output voltages held by circuits 130 and 132 are fed tothe inputs of a summing amplifier 134. Amplifier 134 thus subtracts themaximum amplitude received during each sampling period from the tworeflection patterns 124 and 126 and generates a difference signal whichis fed to the filter, sample and hold circuit 136. The voltage generatedby the amplifier 134 is proportional to the difference in amplitude ofsignals received from the reflection patterns 124 and 126.

This difference signal is filtered and amplified in circuit 136. Thedecoded video signal from the range tracker circuitry shown in FIG. 4 isapplied to the filter, sample and hold circuit 136 for sampling of thedifference signal whenever the range tracking circuitry indicates thatthe pseudorandom coded reflectors 4044 have been positively detected.The resulting signal from the hold circuit 136 is a glideslope errorsignal representative of the vertical position of the aircraft withrespect to the glideslope 14. This glideslope information may bevisually supplied to the aircraft pilot on the display surface 132 inorder to assist him in maintaining the desired glideslope path duringlow visibility landing. For instance, the display may indicate the anglein degrees that the aircraft is above or below the desired glideslopepath.

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art, and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

What is claimed is:

1. An aircraft glideslope detection system comprisa radar system carriedonboard an aircraft for detecting reflected radar signals from aplurality of reflectors spaced at different ranges from the aircraftadjacent an airport runway, each of said reflectors having reflectionpatterns of different angles in the elevation,

range tracking means responsive to said radar system for tracking therange of said reflectors,

means for sampling the amplitudes of said reflection patterns independence upon said range tracking means,

means for comparing the sampled amplitudes of the reflected radarsignals of said reflectors, and

means responsive to said comparing means for generating indications ofthe position of said aircraft with respect to the desired glideslope.

2. The glideslope detection system of claim 1 wherein said radar systemcomprises:

means for transmitting and receiving radar signals from the aircraft,the transmitted radiation pattern being narrower in the azimuth than inthe elevation.

3. The glideslope detection system of claim 2 and further comprising:

antenna means for sweeping the transmitted radiation pattern over anazimuth angle sufficient for airport runway detection during glideslopeapproach.

4. The glideslope detection system of claim 1 wherein said reflectorsare spaced along the side of a runway, one of said reflectors directinga reflection pattern below the glideslope and another of said reflectorsdirecting a reflection pattern above the glideslope, the amplitudes ofsaid reflection patterns being equal along the glideslope.

5. The glideslope detection system of claim 1 wherein indications of theposition of said aircraft with respect to the desired glideslope are notgenerated until said range tracking means has locked onto saidreflectOl'S.

6. The glideslope detection system of claim 1 and further comprising:

a plurality of additional reflectors disposed adjacent the airportrunway for directing radar return signals back to an aircraft accordingto a pseudorandom code to facilitate range tracking.

7. A glideslope detection system comprising:

at least two reflectors located in a spaced apart configuration adjacentan airport runway, one of said reflectors shaped to direct a first radarreflection pattern beneath a desired runway glideslope and the other ofsaid reflectors shaped to direct a second radar reflection pattern abovesaid desired runway glideslope, said first and second reflectionpatterns providing equal amplitude radar returns along said desiredrunway glideslope,

a radar system carried by an aircraft including a range tracker fordetecting the relative radar return signal amplitudes of said first andsecond reflection patterns,

means responsive to said range tracker for sampling the amplitudes ofsaid radar return signals from alternate ones of said reflectionpatterns,

means for comparing the sampled amplitudes of said radar return signals,and

means responsive to said comparing means for generating an indication ofthe position of the aircraft relative to said glideslope.

8. The glideslope detection system of claim 7 wherein the reflectorclosest to the aircraft reflects said first reflection pattern.

97 The glideslope detection system of claim 8 and further comprising:

additional reflectors located adjacent the runway for reflectingaccording to a pseudorandom code to facilitate range tracking. 10. Theglideslope detection system of claim 7 wherein no glideslope indicationis generated until said range tracker locks on to said additionalreflectors.

11. The method of determining glideslope position during an aircraftlanding comprising:

transmitting radar signals from an aircraft during glideslope approachto an airport runway,

reflecting said radar signals from at least two spaced apart locationsadjacent the runway, the reflection pattern from one location beinggenerally directed below the glideslope and the reflection pattern fromthe other location being generally directed above the glideslope,

measuring at the aircraft the amplitudes of said reflection patterns inresponse to range tracking of said spaced apart locations,

comparing said measured amplitudes of said reflection patterns, and

generating in response to said compared measured amplitudes indicationsof the location of the aircraft with respect to the glideslope.

12. The method of claim 1 1 and further comprising:

tracking the range of the aircraft to the runway by detectingreflections from additional locations adjacent the runway which aredisposed according to a pseudorandom code.

13. The method of claim 12 wherein no glideslope position information isgenerated until the range of the additional locations is being tracked.

1. An aircraft glideslope detection system comprising: a radar systemcarried onboard an aircraft for detecting reflected radar signals from aplurality of reflectors spaced at different ranges from the aircraftadjacent an airport runway, each of said reflectors having reflectionpatterns of different angles in the elevation, range tracking meansresponsive to said radar system for tracking the range of saidreflectors, means for sampling the amplitudes of said reflectionpatterns in dependence upon said range tracking means, means forcomparing the sampled amplitudes of the reflected radar signals of saidreflectors, and means responsive to said comparing means for generatingindications of the position of said aircraft with respect to the desiredglideslope.
 1. An aircraft glideslope detection system comprising: aradar system carried onboard an aircraft for detecting reflected radarsignals from a plurality of reflectors spaced at different ranges fromthe aircraft adjacent an airport runway, each of said reflectors havingreflection patterns of different angles in the elevation, range trackingmeans responsive to said radar system for tracking the range of saidreflectors, means for sampling the amplitudes of said reflectionpatterns in dependence upon said range tracking means, means forcomparing the sampled amplitudes of the reflected radar signals of saidreflectors, and means responsive to said comparing means for generatingindications of the position of said aircraft with respect to the desiredglideslope.
 2. The glideslope detection system of claim 1 wherein saidradar system comprises: means for transmitting and receiving radarsignals from the aircraft, the transmitted radiation pattern beingnarrower in the azimuth than in the elevation.
 3. The glideslopedetection system of claim 2 and further comprising: antenna means forsweeping the transmitted radiation pattern over an azimuth anglesufficient for airport runway detection during glideslope approach. 4.The glideslope detection system of claim 1 wherein said reflectors arespaced along the side of a runway, one of said reflectors directing areflection pattern below the glideslope and another of said reflectorsdirecting a reflection pattern above the glideslope, the amplitudes ofsaid reflection patterns being equal along the glideslope.
 5. Theglideslope detection system of claim 1 wherein indications of theposition of said aircraft with respect to the desired glideslope are notgenerated until said range tracking means has locked onto saidreflectors.
 6. The glideslope detection system of claim 1 and furthercomprising: a plurality of additional reflectors disposed adjacent theairport runway for directing radar return signals back to an aircraftaccording to a pseudorandom code to facilitate Range tracking.
 7. Aglideslope detection system comprising: at least two reflectors locatedin a spaced apart configuration adjacent an airport runway, one of saidreflectors shaped to direct a first radar reflection pattern beneath adesired runway glideslope and the other of said reflectors shaped todirect a second radar reflection pattern above said desired runwayglideslope, said first and second reflection patterns providing equalamplitude radar returns along said desired runway glideslope, a radarsystem carried by an aircraft including a range tracker for detectingthe relative radar return signal amplitudes of said first and secondreflection patterns, means responsive to said range tracker for samplingthe amplitudes of said radar return signals from alternate ones of saidreflection patterns, means for comparing the sampled amplitudes of saidradar return signals, and means responsive to said comparing means forgenerating an indication of the position of the aircraft relative tosaid glideslope.
 8. The glideslope detection system of claim 7 whereinthe reflector closest to the aircraft reflects said first reflectionpattern.
 9. The glideslope detection system of claim 8 and furthercomprising: additional reflectors located adjacent the runway forreflecting according to a pseudorandom code to facilitate rangetracking.
 10. The glideslope detection system of claim 7 wherein noglideslope indication is generated until said range tracker locks on tosaid additional reflectors.
 11. The method of determining glideslopeposition during an aircraft landing comprising: transmitting radarsignals from an aircraft during glideslope approach to an airportrunway, reflecting said radar signals from at least two spaced apartlocations adjacent the runway, the reflection pattern from one locationbeing generally directed below the glideslope and the reflection patternfrom the other location being generally directed above the glideslope,measuring at the aircraft the amplitudes of said reflection patterns inresponse to range tracking of said spaced apart locations, comparingsaid measured amplitudes of said reflection patterns, and generating inresponse to said compared measured amplitudes indications of thelocation of the aircraft with respect to the glideslope.
 12. The methodof claim 11 and further comprising: tracking the range of the aircraftto the runway by detecting reflections from additional locationsadjacent the runway which are disposed according to a pseudorandom code.